1. Field of the Invention
This invention relates to a sound insulating building element or structural member.
2. Description of the Prior Art
The insulation of an impervious sheet against air-born sound is mainly determined by the mass, i.e. weight per unit of surface, of the sheet.
A construction consisting of two impervious sheets spaced from each other by an air gap corresponds fundamentally to a mechanical oscillation circuit consisting of two masses interconnected by a spring. The broader the air gap the more resilient is the spring and the greater the sound insulating ability of the composite structure.
This mass-spring system has a marked resonance at a sound frequency, the so-called basic resonance frequency, which is determined by the mass per unit area of the sheets and the breadth of the air gap. At frequencies well below the basic resonance frequency the structure behaves as a single sheet having a mass and a stiffness equal to the total mass and stiffness of the two sheets constituting said structure. At frequencies well above the basic resonance frequency, however, the double-wall provides much higher sound insulation than a single sheet having said total mass. Within a frequency range near the basic resonance frequency the double-structure provides an inferior sound insulation and for this reason it is important to arrange for the basic resonance frequency being as low as possible, suitably below 250 Hz, preferably below 100 Hz. A considerable disadvantage with a double-wall of known construction having good sound insulating properties is that the stiffness and mechanical strength of said construction is very low in relation to the total thickness of the structure and often lower than the stiffness and strength of a homogeneous single sheet having the same weight per surface unit.
If the air gap of the structure having double sheets is substituted by a light but stiff material by means of which the sheets are interconnected, for instance by a binding agent, the stiffness and strength is radically increased, but simultaneously the sound insulation decreases, since the structure will function acoustically almost as a single sheet.